1. Field of the Invention
This invention relates generally to cathode ray tubes, particularly of the single-gun, plural-beam type as used in color video signal reproducing devices, and is more specifically directed to improvements in such cathode ray tubes for compensating or removing beam landing spot distortions due to deflection of the beams for scanning the phosphor screen.
2. Description of the Prior Art
Cathode ray tubes of color video signal reproducing devices, for example, of color television receivers, are being produced with increasingly larger screens. In such cathode ray tubes, for example, of the Trinitron (trademark) type, a single electron gun is disposed within the neck portion of the tube envelope and includes cathodes from which three electron beams corresponding to red, green and blue colors are directed against the color phosphor screen on the panel of the tube envelope which is joined to the neck portion by way of a funnel-shaped portion. The electron gun further includes tubular electrodes which constitute a main focusing lens, preferably of the unipotential type, common to the three beams and by which the latter are focused at the color phosphor screen. A deflection yoke assembly is provided on the tube envelope adjacent the confluence of the neck and funnel-shaped portions and receives horizontal and vertical sweep signals by which the deflection fields are produced for angularly deflecting the beams in respect to the tube axis so as to scan the screen in the horizontal or line-scanning and vertical directions.
When increasing the size of the screen, as aforesaid, it is desirable that the axial dimension of the cathode ray tube, and hence the depth of the cabinet required for containing the same, not be corresponding increased. Therefore, it has been the practice to increase the size of the screen without correspondingly increasing the axial distance from the deflection yoke assembly to the screen, and even while decreasing such axial distance, in which case it will be apparent that scanning of the screen requires wide-angle deflections of the beams from the tube axis. However, by reason of such wide-angle deflections of the beams, distortions occur in the landing spots of the beams on the screen, particularly at the peripheral portions of the latter. Thus, for example, if the electron beams are focused to provide circular landing spots of the beams on the screen when directed at the central portion of the latter, wide-angle deflections of the beams for directing the latter against peripheral portions of the screen will cause the beams to have elongated or otherwise distorted landing spots on such peripheral portions.
In order to avoid the above described beam landing spot distortion in cathode ray tubes, particularly in tubes employing wide-angle deflections of the beams, it has been proposed to provide magnetic or electrostatic quadrupole arrangements by which additional fields are produced for altering the cross-sectional shapes of the beams as the latter are angularly deflected for scanning the screen. However, such previously proposed or existing quadrupole arrangements for correcting the beam landing spot distortion are of low sensitivity, have a relatively high power consumption, require relatively complex operating circuits, and/or give rise to difficulties in manufacturing and assembling the same. Further, some of the proposed arrangements for correcting the described beam landing spot distortion involve additional electrodes that increase the axial length of the electron gun, and thus are somewhat counter-productive in respect to the originally stated purposes for employing the wide-angle deflections of the beams.